1. Field of the Invention
The present invention relates to a variable valve actuating mechanism for a four-stroke internal combustion engine, preferably applicable to a vehicle such as a motorcycle, in which the variable valve actuating mechanism includes a camshaft having a pair of cams for one engine valve, and can selectively use either one of the respective cams for performing an opening/closing operation of the engine valve, depending on engine operating conditions.
2. Description of the Background Art
Conventionally, it has been known to provide a valve actuating mechanism which includes a camshaft, a rocker arm shaft arranged in parallel with the camshaft, and a rocker arm supported on the rocker arm shaft such that the rocker arm is pivotally movable about an axis of the rocker arm shaft, and is also slidably axially movable of the rocker arm shaft. In response to the rotary motion of the camshaft, the rocker arm is brought into contact with either one of the respective cams, and is pivotally rocked to open or close the engine valve. At the same time, the rocker arm is suitably moved in the axial direction thus allowing the selective use of either one of the respective cams for performing opening/closing operation of the engine valve, as disclosed in Japanese published patent application JP-A-2001-20710. In this reference, although the rocker arm is moved in the axial direction using an engine oil pressure depending on an open/closed state of the engine valve, the camshaft pushes the engine valve downwardly by way of the rocker arm (valve open state) and hence, there arises a drawback that it is necessary to increase a force applied to move the rocker arm. Further, when an electric sensor, an electric control or the like is used for moving the rocker arm corresponding to an open/closed state of the engine valve, the arrangement of the valve actuating mechanism per se becomes complicated.